The possibility of identifying user geographical location in a wireless network has enabled a large variety of commercial and non-commercial services. These services include navigation assistance, social networking, location-aware advertising, emergency calls, etc. Different services may have different positioning accuracy requirements imposed by the application. In addition, there may exist regulatory requirements on the positioning accuracy for basic emergency services, an example of which is the FCC E911 service in the US.
In many environments, the position can be accurately estimated by using positioning methods based on GPS (Global Positioning System). Today, some networks may also assist the wireless terminals such as UEs (user equipment) in order to improve the terminal's receiver sensitivity and GPS startup performance such as in A-GPS (Assisted-GPS positioning). GPS or A-GPS receivers, however, are not necessarily available in all wireless terminals. In addition, not all wireless networks necessarily have a possibility to provide or assist in GPS-based positioning. Furthermore, GPS-based positioning may often have unsatisfactory performance in urban and/or indoor environments.
Conventionally, positioning methods based on time difference of arrival measurements (TDOA) have been widely used, for example, in GSM, UMTS and CDMA2000. FIGS. 1a and 1b outline the principle of a downlink observed time difference of arrival (OTDOA) positioning method. Each hyperbola in FIG. 1a illustrates an area with the same level of the reference signal time difference (RSTD) for two base stations. The UE measures the timing differences of multiple base stations. At least three measurements from geographically dispersed base stations with a good geometry are needed to solve for two coordinates of the UE and the receiver clock bias. Thus, to solve for the position of the UE, a precise knowledge of the base station locations and timing are needed. With OTDOA, unlike when measuring the time of arrival (TOA), synchronization between base stations and UEs is not a requirement.
In LTE, to enable positioning and facilitate positioning measurements of a proper quality and for a sufficient number of distinct locations, new physical signals dedicated for positioning—the positioning reference signals (PRS) have been introduced and specific positioning subframes have been agreed upon in 3GPP.
The PRS are transmitted from one antenna port (R6) according to a pre-defined pattern. FIG. 2 illustrates a PRS pattern when one or two physical broadcast channel (PBCH) antennas are in use. In the figure, the squares marked R6 indicate the PRS resource elements (RE) within a block of 12 subcarriers over 14 OFDM symbols, which is a 1 ms subframe with normal cyclic prefix. A set of frequency shifts can be applied to the pre-defined PRS patterns to obtain a set of orthogonal patterns which can be used in neighbour cells to reduce interference on the PRS and thus improve positioning measurements. This allows for modeling an effective frequency reuse of six. The frequency shift, which can also be viewed as vertical shift vshift, can be defined as a function of the Physical Cell ID (PCI) of the cell as follows [3GPP TS 36.211]:Vshift=mod(PCI,6)
The PRS can also be transmitted with zero power, which is one form of muting. Note that the PRS power can be assumed to be constant over an entire positioning occasion [3GPP TS 36.213], including for muting. Thus, if the power is zero, then it is zero in all subframes of the same positioning occasion.
To improve the “hearability” of PRS, that is, to allow for detecting of the PRS from more sites and at a reasonable quality, the positioning subframes have been designed as low-interference subframes. In other words, no data transmissions are allowed in general in the positioning subframes. In synchronous networks as a result, the PRS of a cell are interfered only by the PRS from other cells with the same PRS pattern index, i.e., with the same vshift, and not by data transmissions.
In asynchronous networks, the PRS can still be interfered by data transmissions when the positioning subframes of a cell collide with normal subframes of another cell. The effect can be minimized by partial alignment, i.e., by aligning the beginning of positioning subframes in multiple cells within ½ of a subframe with respect to some time base.
If the UE is unable to detect the PRS for a cell, it will try to detect Common Reference Signals (CRS) and perform RSTD measurements based on the CRS signals. Combining of measurements based on the PRS and the CRS signals in principle can be possible. However, a failure to detect the PRS and then searching for the other signals of the same cell increases the cell detection time and may also degrade positioning measurements. The CRS signals in general have worse hearability than the PRS due to a lower effective frequency reuse of the CRS signals. When two transmit antennas are used for the CRS signals which is typical, the CRS signals typically have an effective frequency reuse of three.
The PRS are transmitted in pre-defined positioning subframes grouped by several consecutive subframes of length NPRS. These pre-defined consecutive NPRS are referred to as a positioning occasion [3GPP TS 36.133]. The positioning occasions occur periodically with a certain periodicity of N subframes, which is the time interval between two positioning occasions. This is illustrated in FIG. 3 in which the first subframes of two positioning occasions are N subframes apart. In LTE, the currently agreed periods for N are 160, 320, 640, and 1280 ms in LTE, and NPRS can be any one of 1, 2, 4, and 6. Note that NPRS can differ from cell to cell.
For the OTDOA positioning, the PRS from multiple distinct locations need to be measured. As such, the UE receiver may have to deal with PRS that are much weaker than those received from the serving cell. Also, without the approximate knowledge of when the measured signals are expected to arrive in time and what is the exact PRS pattern, the UE would need to perform signal search within a large window, which would impact the time and accuracy of the measurements as well as increase the UE's complexity.
As mentioned above, the PRS can be transmitted with zero power. This should then apply for all PRS resource elements within the same subframe over the entire PRS transmission bandwidth. Currently, the way in which the PRS are muted are not specified in 3GPP. Also, no signaling is available to notify the UE on whether the PRS transmissions from a cell are to be muted in a certain subframe or not. However, some solutions have been mentioned or discussed.
One solution that has been put forth is the random muting by cells in which each base station, e.g., eNodeB in LTE, decides either that the PRS transmission opportunities are seized or not and the muting decision is made with some probability. In this implementation, there is no coordination among eNodeB's and the probability is statically configured per eNodeB or per cell. An advantage of this solution is that the decisions are made locally, by each cell, and no signaling among the eNodeBs is necessary.
There are disadvantages to the random muting solution. Real networks are inhomogeneous, with different cell coverage areas and user densities, and possibly different types of base stations. All these imply that setting optimal muting probabilities is a tedious task. Also, the UE does not have information on whether a cell which it is supposed to measure is muted or not further complicating the RSTD measurements and increasing the UE complexity. The optimal configuration of such probabilities may also vary, for example, over each day, and over a week and by cell. These factors make static configurations impractical.
Another solution proposed is to design a limited set of muting patterns and mapping the muting pattern IDs to the PCIs. R1-093793, “Muting for LTE Rel-9 OTDOA Positioning”, 3GPP TSG-RAN WG1 meeting #58bis, October 2009.; R1-092628, “On serving cell muting for OTDOA measurements”, 3GPP TSG-RAN WG1 meeting #57, June 2009. An advantage of the limited muting patterns solution is that given a table of muting patterns and the PCI received in the assistance information, the UE can determine when the PRS are transmitted from the cell of interest without the muting information being explicitly signaled to the UE.
However, this also comes with several disadvantages. One is that the muting patterns need to be either hard coded in UEs, which implies the solution is not suitable for all UEs, or the muting patterns need to be received from the network which would require new signaling. Another disadvantage is that mapping the muting patterns to the PCIs will most likely not result in an optimal muting configuration in non-uniform real networks that may also have a multi-layer structure, i.e. the muting configuration is fixed and thus is impossible to re-optimize unless PCI planning is redesigned for the entire network specifically for positioning which, from the operator's point of view, is most likely to be one of the least desired activity.
The existing solutions have at least the following problems:                1. Poor hearability of the PRS in some scenarios;        2. Permanently defined mapping between PRS transmission pattern and PCIs;        3. No specified way of interference coordination for PRS;        4. Inflexible positioning configuration according to the agreed assistance information; and        5. The positioning solution so far specified in 3GPP does not take into account distributed antenna systems and LTE Advanced (LTE-A) deployment scenarios.        
Each of the identified problems of the existing solutions are discussed in further detail below.
Problem 1: The PRS patterns agreed upon in 3GPP have been designed to model six-reuse in frequency, i.e. the interference comes from every sixth cell in average in a uniformly planned network. However, this may not be sufficient in hierarchical and/or dense network deployments or even in typical real networks where cell shapes are irregular and cell sizes are non-uniform.
Problem 2: As described above, the PRS pattern has been agreed to be a permanently defined function of the PCI. A result is that for the same set of transmitting cells, the interferers are also set and the average level of interference for a stationary UE does not change. This means that the same UE may always experience the same bad interference conditions. Furthermore, the cell ID planning are very likely to be done with respect to reuse factor lower than six due to many considerations other than positioning. The PRS patterns are designed to enable effective six-reuse, while the CRS transmit patterns have effective reuse of three when being transmitted from two antennas which is expected to be a typical scenario.
Problem 3: A possibility of autonomous muting has been agreed upon in 3GPP RAN1. However, it has not been discussed much further and no solution has been agreed. The result is that no signaling is available to inform the UE that the PRS transmissions are muted in a certain cell in certain subframes. This is likely to have a negative impact on the positioning performance.
Problem 4: The agreed upon positioning configuration does define the PRS periodicity and offset of positioning subframes for a given cell. In the existing solutions, it is assumed that all measured cells have the same positioning configuration index IPRS as the serving cell. Without PRS muting, this results in that in synchronous networks, the PRS transmissions always collide in the same cells due to Problem 2.
Problem 5: Deploying distributed antenna systems is an attractive solution to enhance data communication which allows for higher bitrates and lower packet delays. However, from the positioning perspective, there is no gain in simultaneous transmissions of the PRS from distinct locations but using antennas associated with the same PCI. This results in that the UE cannot distinguish whether the signals are transmitted from different locations or have just arrived via multipath being transmitted from the same location. Furthermore, the UE position will then be calculated assuming that the transmitter location is the one associated with PCI, which will result in a greater positioning inaccuracy.
A similar problem occurs with relays type II that are able to decode and retransmit, but cannot be viewed by UEs as separate cells. The UEs could in principle receive the PRS also from such devices, but would require capabilities to figure out that the signals are transmitted by the devices and not base stations, which would further increase the UE complexity.
In dense networks, the PRS are assumed to be hearable from more distant base stations than, for example, the CRS signals. Additionally, with only 504 unique PCIs, it may occur that the PRS from more than one cell with the same PCI can be received in some area. This may occur even more often than with the CRSs, which is expected to be a problem in some scenarios. As an example, in a network with densely deployed base stations to ensure sufficient capacity, the problem is more crucial for PRS than for CRS because CRS may be simply not detectable at that large range where PRS is expected to be detected since PRS have better hearability. Thus the probability of hearing two cells with the same PCI is higher for PRS.
In some wireless networks, beaconing devices or some type of simple devices transmitting PRS may be deployed, which may lead to higher interference on PRS, although such devices may transmit a limited set of signals, e.g. PRS only, and thus do not introduce much interference in general. However, the interference from beaconing devices may be reduced by means of a proper coordination and configuration of transmit signal occasions. The UEs may also need to be able to at least distinguish between the devices and base stations when the devices are reusing the PCIs of the base stations. Furthermore, since such devices in general do not transmit data, the low-interference subframe concept is not really relevant for these types of devices.